Utility vehicle braking

ABSTRACT

A method of brake steering in a four-wheel drive utility vehicle having a driven front axle carrying at least two front wheels, a driven rear axle carrying at least two rear wheels, a powertrain delivering torque to the front axle via a first motor and to the rear axle via a second motor, a controlled clutch arrangement operable to couple the outputs of the first and second motors to vary the distribution of delivered torque between the front and rear axles, and independently operable service brakes on each of the front and rear wheels. The method includes determining a difference in wheel velocity between front and rear wheels; comparing the difference to a threshold value and, when exceeded, automatically engaging the controlled clutch arrangement. A determination is made as to whether or not the vehicle is performing a brake steering manoeuvre and the threshold value is adjusted accordingly. Additional braking force may be applied from independently operable park brakes on the rear wheels in inverse relationship to the level of service brake force applied.

The present invention relates to braking systems for utility vehicles,particularly self-propelled four-wheel drive agricultural machines, andmore particularly to such systems where controlled braking is applied tothe inner wheels of a turning vehicle in order to reduce turning radius.

With the growing size of agricultural machinery, the net power ofagricultural machines such as tractors is also increasing. This alsoresults in an increase in tyre size to transfer the power to the ground.The increasing tyre size also helps to reduce soil compaction which hasnegative impact on crop growth. However, the increased tyre size has theresult that, when the vehicle is turning, parts of the tyres movetowards the area where the engine is installed (engine periphery).Therefore there is a limitation in steering angle which impacts thesteering capability in terms of turning radius, but the minimum turningradius is very important for manoeuvrability (e.g. when turning on aheadland in a field). In addition, stricter exhaust gas emissionrequirements result in more installation space being required in thefront area (e.g. for cooling systems or exhaust emission treatmentsystems), especially in the engine periphery penetrated by the tyresduring steering. Enlarging the installation space in this area,especially the width transverse to driving direction, limits thesteering angle of the tractor. A narrow track width also limits thesteering angle which is an issue for e.g. the US market which has astrong demand for narrow-tracked tractors so that the tyres are able tomove along narrow crop rows.

To mitigate the aforementioned problems it is known to have steeringbrake system. An example of a brake arrangement for steering braking ofa utility vehicle is described in commonly-assigned European patentapplication EP-A-2896540. The vehicle has a cardan brake acting on thefront axle, and separate left and right service brakes on the rearwheels. The left rear service brake is activated by movement of a firstbrake lever and the right rear service brake is activated by movement ofa second brake lever. The left and right rear service brakes and thecardan brake are activated together by movement of both brake levers. Toapply steering braking, only one of the brake levers is applied.

Utility vehicles intended for operation at relatively higher speeds arenormally provided with two separate braking circuits and service brakeson each axle, whereby each circuit is assigned to one axle (in cars, thesplit is more commonly transverse). In consequence, for a utilityvehicle the service brake force ratio for front and rear axle is thereby50/50 which means that each axle is provided with only 50% of the totalavailable braking force under normal operation. In the case of steeringbraking only with the rear axle (as in EP-A-2896540 above), the brakingof only one side results in only half of the rear axle capacity beingused, so only 25% of the total available braking force is applied.Compared to vehicles without front service brakes (which provide 100% ofthe steering force on the rear axle and 50% on one side during steeringbraking) this results in a major disadvantage of insufficient steeringbrake capability.

The main effect used during steering braking is that the braking of therear axle on one side results in that the vehicle being virtuallyrotated about a vertical axis towards the inner curve side (similar to atrack type tractor during differential steering). The front of thetractor is thereby moved out of the track which is given by theAckermann-steering-track. The high weight of modern high horse powertractors is also present at the front and increases the front axle load.In combination, with the bigger wheel sizes on all axles, it is gettingincreasingly difficult to move the inner front wheel inwards duringsteering braking.

Newly developed transmission systems, such as that described inapplicant's pending European patent application EP-A-2935948, comprise acontinuously variable transmission has two branches (one mechanical andone hydrostatic) providing tractive power from the engine to the drivenwheels. The mechanical branch uses gears to provide a mechanical drivebetween the engine and the wheels. The hydrostatic branch uses ahydraulic pump (driven by the engine) to power two hydraulic motorswhich in turn drive the wheels. By varying the ratio of powertransmitted through the mechanical and hydrostatic branches the speedand torque delivered to the driven wheels can be matched to an optimalengine speed whilst maintaining a constant drive between the engine andthe driven wheels. The arrangement of EP-A-2935948 has a first hydraulicmotor is assigned to drive the rear axle and the respective rear wheelswhile the second hydraulic motor is assigned to drive the front axle andthe respective front wheels. Furthermore a clutch is provided so thatthe torque and/or wheel velocity/rotational speed delivered by firsthydraulic motor to drive rear axle and second hydraulic motor to drivefront axle can be adjusted. This has arrangement provides a majorchallenge in terms of steering braking that compared to formerly commondrivelines (wherein a clutch was provided to disconnect the front axlefrom tractive power), as the front wheels are permanently driven at lowvehicle speed at which also steering braking is used.

In accordance with a first aspect of the invention there is provided amethod of brake steering in a four-wheel drive utility vehicle having adriven front axle carrying at least two front wheels, a driven rear axlecarrying at least two rear wheels, a powertrain delivering torque to thefront axle via a first motor and to the rear axle via a second motor, acontrolled clutch arrangement in the connecting shaft operable whenengaged to couple the output of the first and second motors to vary thedistribution of delivered torque between the front and rear axles, andindependently operable service brakes on each of the front and rearwheels, the method comprising;

-   -   determining a difference in wheel velocity between front and        rear wheels;    -   comparing the difference to a threshold value and, when        exceeded, automatically engaging the controlled clutch        arrangement;    -   with the vehicle travelling a substantially straight path,        applying a first level of the threshold value;    -   on the vehicle entering a driven turn, applying a second level        of the threshold value higher than the first;    -   on the vehicle entering a braking turn, applying the service        brakes of the front and rear wheels on the inside of the turn        and applying a third level of the threshold value higher than        the second.

The different levels of the threshold may be determined by the vehiclegeometry (wheelbase, track width, maximum steering angle) and may beadaptively varied as the geometry changes e.g. through the use of stubaxles altering track width, or different tyre sizes affecting maximumsteering angle. As will be described in further detail below, it ispreferred to have the first and second motors driving the respectivelyaxles independently (to reduce mechanical stress in the driveline) butthe automatic engagement of the clutch arrangement for a given disparityin front/rear wheel velocity avoids problems if either of the front orthe rear wheels starts to skid. Applying the different levels ofthreshold enables such skid protection to be triggered at an appropriatepoint depending on whether the vehicle is driving straight, driving acurved path, or performing a braking turn.

Where the vehicle is additionally provided with independently-operableparking brakes on each of the rear wheels the method may furthercomprise, on entering a turn applying also, and to a predetermined levelof braking force, the parking brake on the rear wheel on the inside ofthe turn. Suitably, the level of parking brake braking force applied toa rear wheel on entering a turn is substantially proportional to thelevel of service brake braking force applied to the same wheel.

Preferably, the level of service brake braking force applied during aturn may be determined by the level of pressure exerted by a user of thevehicle on a brake control of the vehicle.

Also in accordance with the present invention there is provided adriveline for a four-wheel drive utility vehicle comprising:

-   -   a driven front axle carrying at least two front wheels;    -   a driven rear axle carrying at least two rear wheels;    -   a powertrain delivering torque to the front axle via a first        motor and to the rear axle via a second motor;    -   a controlled clutch arrangement operable when engaged to couple        the outputs of the first and second motors to vary the        distribution of delivered torque between the front and rear        axles;    -   independently operable service brakes on each of the front and        rear wheels;    -   first and second sensors respectively disposed to determine        front and rear wheel velocity;    -   a turn sensor disposed to measure a turning angle of the        vehicle;    -   data storage holding data defining first, second and third        levels of a threshold value; and    -   a control system coupled with the powertrain, sensors, data        storage, clutch arrangement and service brakes, and configured        to:    -   determine a difference in wheel velocity between front and rear        wheels from the first and second sensors;    -   compare the difference to the threshold value and, when        exceeded, automatically engage the controlled clutch        arrangement;    -   determine from the turn sensor when the vehicle travels a        substantially straight path, and apply the first level of the        threshold value;    -   determine from the turn sensor when the vehicle enters a driven        turn, and apply the second level of the threshold value, which        second level is higher than the first; and    -   determine at least partly from the turn sensor when the vehicle        enters a braking turn, apply the service brakes of the front and        rear wheels on the inside of the turn and apply the third level        of the threshold value, which third level is higher than the        second.

Preferably the driveline further comprises independently-operableparking brakes on the rear wheels, with the control system beingconfigured on detecting that the vehicle is entering a turn to applyalso, and to a predetermined level of braking force, the parking brakeon the rear wheel on the inside of the turn, suitably with a level ofparking brake braking force that is substantially proportional to thelevel of service brake braking force applied to the same wheel.

Further features of the driveline are recited in the attached claims, towhich attention is now directed, and the disclosures of which areincorporated herein by reference.

Further in accordance with the present invention there is provided autility vehicle including a driveline as set forth above. The utilityvehicle may further comprise a geographical positioning system coupledwith the control system, with the control system being configured to notimplement the above-recited method of brake steering on determinationthat the vehicle is outside of a predetermined geographical area.

Further advantages of the invention will become apparent from readingthe following description of specific embodiments with reference to theappended drawings in which:—

FIG. 1 is a representation of a utility vehicle, in the form of atractor, suitably provided with a braking system embodying the presentinvention;

FIG. 2 is a schematic representation of the driveline arrangement of thetractor of FIG. 1;

FIG. 3 is a schematic circuit diagram of a first embodiment of apneumatically operable brake system for the tractor of FIG. 1;

FIG. 4 is a schematic circuit diagram of a further embodiment of apneumatically operable brake system for the tractor of FIG. 1;

FIG. 5 represents a method of brake steering as may be effected by thetractor of FIG. 1; and

FIG. 6 represents a relationship between applied service and park brakeforces.

Referring to FIG. 1, a utility vehicle in the form of a tractor 10 isshown having a cab 12 and an engine compartment 14. A chassis 16 whichis partly visible connects a front wheel suspension and steeringassembly (indicated generally at 18) and a rear axle assembly (indicatedgenerally at 20). A vehicle control system (represented schematically at62) is coupled to receive data from a number of sensors 11: such datamay include (but is not limited to):

-   -   gross weight of the vehicle;    -   amount of front and/or rear ballasting carried by the vehicle;    -   weighting information pertaining to a towed or carried        implement;    -   extent of measured wheel-slip for one or more wheels of the        vehicle;    -   tyre pressure in one or more tyres on respective wheels of the        vehicle;    -   angle of turn directed by a user of the vehicle;    -   current speed of the vehicle;    -   ambient conditions external to the vehicle.

Further inputs to the control system 62 may come from a user-operableinput device such as a touchscreen display and input device 13positioned in the vehicle cab 12, and a geographical positioning system15 for the vehicle.

Referring additionally to the driveline arrangement of FIG. 2, a primemover such as an internal combustion engine 22 drives an input shaft 24of a gearbox/transmission unit 26 via a flywheel 28. The transmission 26drives front and rear output (axle drive) shafts 32, 30 to providepropulsive drive to the respective axle assemblies 18, 20. In additionto providing propulsive drive, the transmission 26 also provides driveto a rear power take-off drive shaft 64.

The transmission input shaft 24 is connected at its inboard end to aplanetary gear assembly indicated generally at 148. The purpose of theplanetary gear assembly 148 is to split the torque provided by the inputshaft 24 between a mechanical branch indicated generally at 150 and ahydrostatic branch indicated generally at 152. On the opposite side ofthe planetary gear assembly to the input shaft 24 is the rear power takeoff shaft 64.

The hydrostatic branch 152 drives a hydraulic pump 140. The mechanicalbranch 150 is connected to the front axle drive shaft 32 and rear axledrive shaft 30 as follows. Torque is transmitted from the mechanicalbranch 150 of the planetary gear assembly 148 to the rear axle driveshaft 30 via a rear axle gear 154. Mounted on the same shaft as the rearaxle drive gear 154 is an intermediary gear 156 which in turn drives afront axle drive gear 158 which selectively drives the front axle driveshaft 32. A first clutch 160 is provided to selectively engage anddisengage the front axle drive shaft 32 from the rear axle drive shaft30 or to control the ratio of torque distribution between the two axles.This allows grip to be optimised dependent on the ground conditions.

In addition to the mechanical drive path described above, the hydraulicpump 140 is hydraulically connected (not shown in FIG. 2 for clarity) toa first hydraulic motor 142 which is driveably connected to the rearaxle drive shaft 30 in order to provide hydraulic drive to the rearwheels. The hydraulic pump 140 is also connected in parallel to a secondhydraulic motor 144 in order to provide hydraulic drive to the frontaxle drive shaft 32 as follows. The motor 144 is driveably connected tothe front axle drive shaft 32 via first and second hydraulic motor gears162, 164. A second clutch 166 allows the second hydraulic motor 144 tobe selectively engaged and disengaged from the front axle drive shaft32. This allows hydraulic drive to be provided to the front axle driveshaft 32 by the second hydraulic motor 144 in addition to, oralternatively to, the drive delivered to the front axle drive shaft 32from the rear axle drive shaft 30 via the intermediary and front axledrive gears 156, 158, depending on the extent of engagement of the firstclutch 160.

Second hydraulic motor 144 is connected to front axle drive shaft 32 bygears 162, 164 having a high transmission ratio. This allows motor 144to provide high torque at a limited, lower range of vehicle speeds.Consequently, at higher vehicle speeds, the motor 144 may bedisconnected from the driveline via second clutch 166. Due to thelayout, the first hydraulic motor 142 is provided for delivering lowertorque but over the full range of vehicle speeds. However, incombination, both motors 142, 144 enable the transmission to provide afull transmission output power with variable torque, variable vehiclespeed and variable driving direction over a full range of vehiclespeeds.

Rear axle drive output shaft 30 drives the vehicle rear axle left andright driveshafts 34L, 34R via rear axle differential 36. Note thedesignations “front”, “rear”, “left” and “right” as used herein aretaken from the point of view of a user/driver sat facing forward in thecab 12. The rear axle assembly 20 further comprises left and right rearaxle service brakes 38L, 38R (with respective park brakes 40L, 40R),left and right rear axle final drives 42L, 42R, and left and right rearwheels 44L, 44R. As shown, the service and park brakes may share acommon set of brake disks 39L, 39R, with the service brake beingspring-biased to the open position and the park brake spring-biased tothe closed position. Such an arrangement is described in e.g. Germanutility model DE9204417U1.

In like manner, front axle drive output shaft 32 drives the vehiclefront axle left and right driveshafts 46L, 46R via cardan shaft 48 andfront axle differential 50. The front axle assembly 18 further comprisesleft and right front axle service brakes 52L, 52R, left and right frontaxle final drives 54L, 54R, and left and right front wheels 56L, 56R.The controlled clutch arrangement, comprising the first and secondclutches 160, 166 under the direction of the vehicle control system 62(described further below), the drive to the vehicle front wheels 56L,56R may be selectively engaged or disengaged, or engaged with acontrollably variable degree of clutch slippage to enable the engineoutput torque delivered to the axle assemblies 18, 20 to be controllablyvaried.

The first clutch 160 is also provided to control the wheel velocity orrotational speeds of the axle assemblies 18, 20 to avoid malfunction ofthe transmission 26. With reference to FIG. 1, the wheel velocity w isthe velocity of a wheel in the contact point with the ground G and alongthe ground (radially). Considering a known wheel diameter, therotational speed n of the wheel can be calculated and based on that thewheel velocity w can be determined by measuring the rotational speed nat any shaft in the driveline which is connected via a fixed, constantratio to one of the wheel axles 18, 20. If the vehicle is equipped withdifferent tyre sizes (requiring an overall gear ratio between front axleand rear axle, the wheel velocity wF (for front axle) and wR (for rearaxle) should be equal under ideal conditions while due to the divergingtyre diameter, rotational speed nF of front axle and nR of rear axle aredifferent. But as the respective gear ratios in the driveline from atransmission 26 to a respective front or rear axle 18, 20 are known,both rotational speeds nF, nR and also wF, wR can be monitored bymeasuring the rotational speed n at any shaft in the driveline which isconnected to the respective wheel axles 18, 20.

Control system 62 is permanently monitoring the signals coming fromspeed sensors S1 and S2. Sensor S1 is connected to the rear axle driveshaft 30 with a fixed ratio so that sensor S1 provides a signalindicative of the rotational speed of rear axle drive shaft 30 (referredto as n1), rear axle left and right driveshafts 34L, 34R and therebyleft and right rear wheels 44L, 44R. Sensor S2 is connected to the frontaxle drive shaft 32 with a fixed ratio so that sensor S2 provides asignal indicative of the rotational speed of front axle drive shaft 32(referred to as n2), front axle left and right drive shafts 46L, 46R andthereby left and right front wheels 56L, 56R.

The necessity of monitoring speeds n1 and n2 in connection with thespecial transmission design described in applicant's pending Europeanpatent application EP-A-2935948 is explained with reference to FIGS. 1and 2. The pump 140, the first and second motors 142, 144 are providedwith adjustment means (not shown), the adjustment means being operableby an actuator to vary the operating angle of the first and secondmotors 142,144, respectively. Advantageously, the provision of aseparate adjustment means for each of the motors 142,144 allows thespeed and torque output of the motors to be independently and flexiblycontrolled. This offers significant advantages in terms of vehiclecontrol and efficiency. For example, different pivot angles can beprovided for each motor allowing one motor can be pivoted to zerodisplacement (represented by a pivot angle of 0 degrees or 45 degrees,depending on specification) while the torque or speed output of theother motor is further adjusted. Furthermore, as one motor can bepivoted to zero displacement while the second motor is at a displacementabove zero, the first motor can be disconnected by clutch 166.Accordingly, the control system 62, and particularly the relationshipbetween the displacement of first and second motor 142, 144, can beadapted. This allows the transmission system to be readily configuredfor different applications. It must be understood that the adjustmentmeans is controlled depending on the vehicle speed v and that theadjustment means is designed so that for each vehicle speed v, adisplacement (pivot angle) for pump 140, first and second motor 142, 144is predetermined. This means that at e.g. a vehicle speed of say 8 kph(kilometre per hour) pump 140, first and second motor 142, 144 areadjusted to first set of displacements while at a speed say 25 kph, pump140, first and second motor 142, 144 are adjusted to second set ofdisplacements. For each vehicle speed v a predetermined displacement forpump 140, first and second motor 142, 144 is assigned which cannot beadapted in relation to each other.

When the tractor 10 is driven at constant vehicle speed v over ground G,the wheel velocity for front and rear axle wF and wR should be the sameif wheel-slip is equal for both axles (in simple terms, the ground Gunder the wheels shows the same condition). A wheel-slip of 10% (whichis typical for soft ground like fields) would mean that the wheelvelocity for front and rear axle wF and wR is 10% higher than thevehicle speed.

Looking now at the rotational speed of the axles of a tractor with bothfront axle and rear axle being equipped with tyres of the same size (interms of the outer diameter), the rotational speed nF and nR is equal,so the rotational speed n1 determined by sensor S1 and the rotationalspeed n2 determined by sensor S2 are also equal. If a tractor withdifferent tyre sizes (in terms of the outer diameter) is regarded,rotational speed nF and nR would be different due to overall gear ratiobetween front axle and rear axle. So a comparison of the rotationalspeeds n1 and n2 to arrive at the deviation/difference in wheel velocityor rotational speed of front and rear axle wF and wR would requireinclusion of the overall gear ratio. As this is standard engineeringknowledge, the following description considers equal tyre sizes for bothaxles 18, 20 so that the overall gear ratio need not be applied andconsidered. Based on that, some operating conditions of the tractor 10are now described in detail:

Driving Straight Ahead with Both Axles on Same Ground:

When driving straight ahead, the wheel velocity wF and wR and (for equaltyre sizes) rotational speed nF and nR of front and rear axle 18, 20should be equal if both axles drive on similar ground G. In thiscondition the clutch 160 is disengaged so that first hydraulic motor 142is driving the rear axle 20 via rear axle drive shaft 30 without anyconnection to front axle drive shaft 32 which is driven by second motor144 to drive front axle 18 (when clutch 166 is engaged). This conditionis preferred as independent drive for the axles is provided andtorsional stresses in the driveline can be avoided. However, there aresituations where the engagement of clutch 160 is advantageous.

Driving Straight Ahead with Both Axles on Different Ground:

If the front axle 18 or one wheel 56L, 56R of the front axle 18 is nowdriving on sandy or frozen ground (with the rear axle still driving onhard soil) with the clutch 160 is disengaged, the front axle would startspinning (as the torque supplied cannot be transferred to the ground).In terms of the wheel velocity, this means that the wheel velocitywF/rotational speed nF of the front axle 18 would increase considerablycompared to the wheel velocity wR/rotational speed nR of the rear axleor, in other words, the wheel velocity difference Dw=wF−wR or Dn=nF−nR(being ideally zero) would increase. As a consequence, the fluidsupplied by pump 140 would be completely consumed by second hydraulicmotor 144 (due to missing resistance/torque support) so that firsthydraulic motor 142 would stand still. In other words, when looking atthe pressure in the supply circuit for pump 140, and first and secondmotors 142, 144, due to the missing torque support at second motor 144,the oil would flow at a minimal pressure level (nearly pressureless) insecond motor 144 assigned to front axle 18. As both motors 142, 144 areconnected in the same hydraulic circuit (in parallel), the same minimalpressure level would impinge first motor 142 assigned to rear axle 20.As a result first motor 142 cannot supply torque. With front axle 18spinning on sandy or iced ground and rear axle 20 standing still, thetractor would just slow down and stop. Changing the vehicle speed toadjust speeds and torque supplied by motors 142, 144, may help to acertain degree but not on ground which is glassy frozen. Furthermorereducing speed is not advantageous during agricultural work (e.g.ploughing or seeding) as the work result may suffer.

Opening clutch 166 without second motor 144 adjusted to zerodisplacement would have the same result. This condition is only intendedfor higher speeds at which the adjustment means changing second motor144 to zero displacement and the clutch 166 is disengaged while firstmotor 142 is adjusted to displacement above zero.

To avoid the unintended stand still of the tractor on sandy or icedground, the control unit 62 is permanently adjusting the engagement ofthe clutch 160 as explained below:

During Straight Ahead Drive

The monitoring process includes that the determined rotational speeds ofsensors S1 and S2, are permanently compared to detect the wheel velocitydifference/rotational speed difference. If during straight ahead drivingthe value of the wheel velocity difference/rotational speed differenceexceeds a first wheel velocity difference threshold value say 5%(meaning that wheel velocity wF is 5% higher than wheel velocity wR) thecontrol unit controllably engages clutch 166 until the first wheelvelocity difference threshold value is undercut again. If the front axlespins on a sandy surface, this would result in that the clutch isfurther engaged so that the second motor 144 is drivingly connected tothe rear axle which can support the torque to avoid spinning. This wouldalso keep first motor under supply to drive the rear axle. The vehiclewould not be forced into stand still then. This method keeps independentdrive of both axles 18, 20 upheld most of the time to avoid torsionalstresses in the driveline.

During Turning

During a driven turn (determined by steering sensor S3), a second wheelvelocity difference threshold value is considered. Based on the factthat during a turn, due to Ackermann steering constraints, the steeredfront wheels roll on a greater curve radius (path) so that they have tospeed up to pass the curved path at the same time compared to the rearwheels. So during turning, the clutch control unit 62 would consider asecond wheel velocity difference threshold value which may be 15 to 20%.This higher level for the velocity difference threshold enables thevehicle to pass the curve but the spinning prevention is still active,so that in the case when the front wheels drive via icy surface in thecurve, the system can still react. Furthermore, the drag capability(that the front wheels support the turn)

During Steering Braking

During a braking turn (determined by steering sensor S3 and theactivation of the steering brake), a third wheel velocity differencethreshold value is considered. In case of steering braking, the innersteered wheel is braked while the steered outer wheel should support thesteering brake by speeding up to further drag the vehicle into thecurve.

So during turning, the clutch control unit 62 would consider a thirdwheel velocity difference threshold value which may be 30%. This enablesthat the vehicle can pass the curve and steering brake but the spinningprevention is still active, so that in the case when the front wheelsdrive via icy surface in the curve, the system can still react

The values for second wheel velocity difference threshold and thirdwheel velocity difference threshold value thereby depend on the geometryof the vehicle (wheel distance or wheelbase, maximum steering angle α,track width) as these parameters influence the path driven during a turnwhich themselves influence the wheel speed differences which must beallowed:

-   -   For a vehicle with a smaller wheel distance (distance between        the axles in driving direction) the threshold value may be        reduced (assuming that no other geometry changes).    -   For a vehicle with a smaller track width (distance between        wheels of an axle transverse to driving direction) the threshold        value may be reduced (assuming that no other geometry changes).    -   For a vehicle with a reduced maximum steering angle α the        threshold value may be reduced (assuming that no other geometry        changes)

While the wheel distance may not be easily changed on a vehicle, thetrack width can be adapted with special axle arrangements (known as stubaxles in the USA) so that also the steering angle may be limited.Alternately, the steering angle may be limited by attaching a frontloader. The values for the velocity difference thresholds, which aresuitably held in a data storage device 62 a of the control system 62,may be adapted by the driver via the input device 13, or by the controlsystem 62 recognizing these configurations.

FIG. 3 shows a schematic circuit diagram of a pneumatically operablebrake system for the tractor 10 of FIG. 1. A first brake circuit C1 isprovided for activating left and right rear service brakes 38L, 38Rindividually for application to left and right rear wheels 44L, 44Rrespectively. This first brake circuit C1 comprises a first pressurecontrol valve 70 and supply lines L3, L3 a, L3 b, L4, L4 a and L4 b.Left and right rear service brakes 38L, 38R are operated by respectiveleft and right associated brake relay valves 72L, 72R. Lines L3 a and L3b are linked by a shuttle valve 74 which passes the greater pressure ineither of the lines to proportional valve 76 in the control circuit forrear park brakes 40L, 40R (described below).

A second brake circuit C2 is provided for activating left and rightfront service brakes 52L, 52R individually for application to left andright front wheels 56L, 56R respectively. This second brake circuit C2comprises a second pressure control valve 78 and supply lines L1, L1 a,L1 b, L2, L2 a and L2 b. Left and right front service brakes 52L, 52Rare operated by respective left and right associated brake relay valves80L, 80R.

The service brakes 38L, 38R, 52L, 52R are activated by the driver by twolevers, such as two foot pedals 82L, 82R. Left or first foot pedal 82L,when pressed opens a left rear brake valve 84L and a left front brakevalve 86L. Right or second foot pedal 82R when pressed opens a rightrear brake valve 84R and a right front brake valve 86R. Pedals 82L, 82Rwill activate a piston or pistons 88 of a cylinder or cylinders (notshown) which activate the first and second pressure control valves 70,78. Movement of either or both of the pedals 82L, 82R will activate bothof the first and second pressure control valves 70, 78.

The service brakes 38L, 38R, 52L, 52R are connected to a fluid supply 90such as a compressor, or air chamber via their respective brake relayvalves 72L, 72R, 80L, 80R. When neither pedal 82L nor pedal 82R ispressed, the brake relay valves 72L, 72R, 80L, 80R are in a closedposition which means that the brakes 38L, 38R, 52L, 52R are notactivated. Each brake circuit C1, C2 is connected to a separate fluidreservoir of the fluid supply 90. The first brake circuit C1 includingfirst control valve 70 is connected to fluid supply 90 a, and the secondbrake circuit C2 including second control valve 78 is connected to fluidsupply 90 b.

The first control valve 70 switches left associated rear brake valve72L, or right associated rear brake valve 72R, or both of them, to anopen position via left and/or right brake valves 84L, 84R. When theassociated rear brake valve 84L, 84R is open, the respective rearservice brake 38L, 38R is activated. Associated rear brake valves 84L,84R are connected in parallel.

The second control valve 78 switches left associated front brake valve80L, or right associated front brake valve 80R, or both of them, to anopen position via left and/or right brake valves 86L, 86R. When theassociated front brake valve 86L, 86R is open, the respective frontservice brake 52L, 52R is activated. Associated front brake valves 86L,86R are connected in parallel.

When neither of the pedals 82L, 82R is moved, the front and rear brakevalves 86L, 86R, 84L, 84R are biased to a closed position.

During road operation, legal regulations require simultaneous braking onboth sides. Therefore, the two foot pedals 82L. 82R must be connected bya locking means (not shown) so that only simultaneous movement ispossible. This locking means can be provided as a pin which engagesthrough the two foot pedals 82L, 82R and which is operated by thedriver. Alternatively, the locking means may be engaged automatically,e.g. above a certain vehicle speed. Such locking means are described inapplicant's published patent application WO2010/066864.

When both foot pedals 82L, 82R are pressed together, piston 88 activatesfirst and second control valves 70, 78 and at the same time the frontand rear brake valves 86L, 86R, 84L, 84R are opened. Air flows from thefluid supply 90 b, along line L1, through second control valve 78, alongline L2 through the front brake valves 86L, 86R and through to therespective front brake valves 80L, 80R which switches valves 80L, 80R toan open position. Air can then flow from the fluid supply 90 b alongline L1 and parallel lines L1 a and L1 b to the respective front servicebrakes 52L, 52R. At the same time, air also flows from the fluid supply90 a, along line L4 through first control valve 70, along line L3through left and right brake valves 84L, 84R and along lines L3 a, L3 bthrough to associated brake relay valves 72L, 72R which switch valves72L, 72R to an open position. Air can then flow from air supply 90 athrough line L4, through lines L4 a and L4 b to activate rear servicebrakes 38L, 38R.

If the driver wishes to apply the brakes on one side only, for examplethe left front service brake 52L and left rear service brake 38L to helphim steer left around a bend, the driver pushes the left foot pedal 82Lonly, after disengaging the above-mentioned locking means connecting thetwo foot pedals 82L, 82R during road operation.

The rear park brakes 40L, 40R are controlled by a park brake controlcircuit C3 connected to a further separate reservoir 90 c of fluidsupply 90. In conventional operation, a park brake control valve 92,activated by a hand brake lever, is operable to connect the fluidsupply, via a relay valve 94 and respective left and right rear solenoidvalves 96L, 96R, to release the park brakes 40L, 40R (and dosing parkbrake force). The solenoid valves 96L, 96R are spring biased to theoperating position shown in which the output of relay valve 94 isconnected to the park brakes 40L, 40R.

To enhance the brake steering operation, the park brakes 40L, 40R areused to supplement the braking pressure applied by the rear servicebrakes 38L, 38R. The fluid supply 90 from reservoir 90 c is connected asa further input to proportional valve 76 (along with the output fromshuttle valve 74 in circuit C1) with the proportional valve out put online L5 being connected to the left and right rear solenoid valves 96L,96R. Operating one of the rear solenoid valves (e.g. left rear solenoidvalve 96L) disconnects the respective park brake 40L from the output ofrelay valve 94 and instead connects it to the output of proportionalvalve 76 on line L5. The effect of the proportional valve 76 is toreduce the opening pressure applied to the park brake 40L as the closurepressure on the corresponding service brake 38L is increased (So fluidpressure applied on the park brake is substantially inverselyproportional to that applied on the service brake) such that the brakingforce applied by the park brake is substantially proportional to thatapplied by the service brake. This proportional application of the parkbrake during brake steering reduces the ground damage that may otherwiseoccur if maximum park brake force were applied regardless of servicebraking force (as would be the case with the above-mentioned combinedpark and service brake arrangement of DE9204417U1).

FIG. 4 shows an alternative embodiment of the invention in which thepedal-operated front brake valves 86L, 86R are replaced bysolenoid-operated valves 98L, 98R connected between the line L2 outputof second control valve 78 and the respective front brake valves 80L,80R operating left and right front service brakes 52L, 52R. Othercomponents of the arrangement of FIG. 4 are the same as in FIG. 3, aredenoted by the same reference numerals, and will not be furtherdescribed.

The brake circuit shown in FIG. 4 may be adapted to a full electronicbraking system (brake by wire) wherein the left rear brake valve 84L andthe right rear brake valve 84 k are not directly connected to two footpedals 82L, 82 k. Instead, solenoid valves are used to activate the rearservice brakes 38L, 38R. The movement of the two foot pedals 82L, 82Rmay then be measured by sensors and forwarded to control unit 62, whichin turn controls the solenoid valves. Furthermore, such a brake systemmay be provided with only one foot pedal. A further activation means maybe provided so that the driver can activate steering brake operation orthe system may automatically activate steering brake depending on sensedparameters (vehicle speed, field/road operation, etc.). Furthermore thepark brake control valve 92 may be also solenoid valves (connected withcontrol system 62) which is operable to connect the fluid supply, via arelay valve 94 and respective left and right rear solenoid valves 96L,96R, to release or activate the park brakes 40L, 40R. In such anarrangement, the brake force of the park brake under normal operatingcondition, cannot be controlled by the driver, only ON/OFF condition ispossible.

Whilst the above arrangement described is intended for use as apneumatic circuit, it is envisaged that the above described brakearrangement can also be operated hydraulically.

FIG. 5 represents a method of brake steering as may be effected by thetractor 10 of FIG. 1 under the direction of the control system 62. Fromstarting at 100, at step 102 a determination is made as to whether thetractor is travelling substantially straight. If so, at step 104 thewheel velocity difference value is set to the first and lowest value(LEVEL=1).

If the tractor is not travelling substantially straight, at step 106 adetermination is made as to whether the tractor is travelling a driventurn. If so, at step 108 the wheel velocity difference value is set tothe second and intermediate value (LEVEL=2).

If the tractor is neither travelling substantially straight nor a driventurn, which is indicative of steering braking, at step 110 the wheelvelocity difference value is set to the third and highest value(LEVEL=3) for a steering braking turn, the service brakes 38 are appliedon one or both of the wheels on the inside of the turn at step 112 and(as described above) optionally the parking brake 40 of the rear wheelon the inside of the turn may be applied at step 114.

Following the selection of the first or second wheel velocity differencethreshold values at steps 104 and 108, or the application of the servicebrakes at step 112, a check is made at step 116 as to whether thecurrent magnitude of wheel velocity difference value exceeds thecurrently selected threshold and, if so, at step 118 the first clutch160 is engaged.

As described above and represented in FIG. 6 (with the pedal stroke ofpedals 82L or 82R depicted on the horizontal axis while the brakeforces/fluid pressures are depicted on the vertical axis), thecontrollably varied level of braking force applied by the parking brake(as depicted with graph G1) is preferably substantially proportional tothe level of applied service brake force (as depicted with graph G2)while the fluid pressure applied on the park brake (as depicted withgraph G3) is substantially inversely proportional to that applied on theservice brake (as depicted with graph G4). The graphs G1 and G2 may showequal values but are depicted with a small offset for clarity reasons.The pedal stroke shown on the horizontal axis starts with 20% whichmeans that before, no brakes are actuated but e.g. the brake lights areanyway switched on, e.g. at a stroke of 10% before the vehicle isbraked.

Referring back to FIG. 5, the second and third wheel velocity differencevalues between the front and rear axles may be set to a fixed amount,e.g. 15% and 30%. Alternatively, an optional step may comprise activelyadjusting the wheel velocity difference values based on external factorsidentified by the sensors 11, such as:

-   -   gross weight of the vehicle;    -   amount of front and/or rear ballasting carried by the vehicle;    -   weighting information pertaining to a towed or carried        implement;    -   extent of measured wheel-slip for one or more wheels of the        vehicle;    -   tyre pressure in one or more tyres on respective wheels of the        vehicle;    -   angle of turn directed by a user of the vehicle;    -   current speed of the vehicle;    -   ambient conditions external to the vehicle.

If sensor input indicates the vehicle is heavier at the rear, thecontrol system 62 may not activate the front service brake, while whenthe vehicle is heavier at the front, the system may not activate theadditional brake force supplied by park brake. This situative brakecontrol enables an efficient operation avoid excessive brake force toreduce soil damages. An ABS sensor giving wheel velocity to determinetyre slip may then be used for control of brakes and clutch.

As brake steering should only be used in field conditions, and not onthe road, a further determination may be made at 120 prior to the startof a turn at 100 as to whether the vehicle is in a “safe” geographicallocation, namely a field. This determination may be made automaticallyin control system 62 on the basis of input from the geographicalpositioning system 15 (FIG. 1).

In the foregoing the applicants have described a method of brakesteering in a four-wheel drive utility vehicle having a driven frontaxle carrying at least two front wheels, a driven rear axle carrying atleast two rear wheels, a powertrain delivering torque to the front axlevia a first motor and to the rear axle via a second motor, a controlledclutch arrangement operable to couple the outputs of the first andsecond motors to vary the distribution of delivered torque between thefront and rear axles, and independently operable service brakes on eachof the front and rear wheels. The method comprises determining adifference in wheel velocity between front and rear wheels; comparingthe difference to a threshold value and, when exceeded, automaticallyengaging the controlled clutch arrangement. A determination is made asto whether or not the vehicle is performing a brake steering manoeuvreand the threshold value is adjusted accordingly. Additional brakingforce may be applied from independently operable park brakes on the rearwheels in inverse relationship to the level of service brake forceapplied.

From reading of the present disclosure, other modifications will beapparent to those skilled in the art. Such modifications may involveother features which are already known in the field of vehicle drivelineand braking systems and component parts therefore and which may be usedinstead of or in addition to features described herein.

1. A method of brake steering in a four-wheel drive utility vehiclecomprising: carrying at least two front wheels on a driven front axle;carrying at least two rear wheels on a driven rear axle; deliveringtorque through a powertrain to the front axle via a first motor and tothe rear axle via a second motor; determining a difference in wheelvelocity between the front and the rear wheels; comparing the differenceto a threshold value and, when exceeded, engaging a controlled clutcharrangement in a connecting shaft, coupling an output of the first andthe second motors, and varying distribution of the delivered torquebetween the front and the rear axles; with the vehicle travelling asubstantially straight path, applying a first level of the thresholdvalue to independently-operable service brakes on each of the front andrear wheels; with the vehicle entering a driven turn, applying a secondlevel of the threshold value higher than the first to the service brakeson the front and the rear wheels; with the vehicle entering a brakingturn, applying a third level of the threshold value higher than thesecond, to the service brakes on the front and the rear wheels adjacentto an inside of the turn.
 2. The method of brake steering as claimed inclaim 1 further comprising: with the vehicle entering the braking turnadditionally applying a predetermined level of braking force toindependently-operable parking brakes on each of the rear wheelsadjacent to the inside of the turn.
 3. The method of brake steering asclaimed in claim 2, wherein the level of braking force applied to theparking brakes of the rear wheels on entering the braking turn issubstantially proportional to the level of braking force applied to theservice brakes of the same rear wheels.
 4. The method of brake steeringof claim 1, wherein the level of braking force applied to the servicebrakes during the braking turn is determined by the level of pressureexerted on a user-operable brake control of the vehicle.
 5. The methodof brake steering of claim 1, wherein at least one of the second andthird levels of the threshold value is determined at least partially onthe vehicle wheelbase.
 6. The method of brake steering of claim 1,further comprising adapting at least one of the second and thirdthreshold value levels in response to changes to a track width of thevehicle.
 7. The method of brake steering of claim 1, further comprisingadapting at least one of the second and third threshold value levels inresponse to changes to a maximum steering angle of the vehicle.
 8. Themethod of brake steering of claim 1 wherein a ratio of braking forceapplied to the service brakes of the front wheels and the breaking forceapplied to the service breaks of the rear wheels is controllably variedbased on one or more factors comprising: gross weight of the vehicle;amount of front and/or rear ballasting carried by the vehicle; weight ofa towed or carried implement; measured wheel-slip for the one or morewheels of the vehicle; tyre pressure in one or more tyres on therespective wheels of the vehicle; angle of the turn of the vehicle;current speed of the vehicle; ambient conditions external to thevehicle.
 9. A driveline for a four-wheel drive utility vehiclecomprising: a driven front axle carrying at least two front wheels; adriven rear axle carrying at least two rear wheels; a powertraindelivering torque to the front axle via a first motor and to the rearaxle via a second motor; a controlled clutch arrangement configured to,when engaged; couple outputs of the first and the second motors and varydistribution of the delivered torque between the front and the rearaxles; independently operable service brakes on each of the front andthe rear wheels; first and second sensors respectively configured todetermine the front and the rear wheel velocity; a turn sensorconfigured to measure a turning angle of the vehicle; a non-transitorydata storage unit holding data defining a first, second and third levelsof a threshold value; and a control system coupled with the powertrain,the sensors, the data storage unit, the clutch arrangement and theservice brakes, configured to determine a difference in wheel velocitybetween the front and the rear wheels from the first and the secondsensors; compare the difference to the threshold value and, whenexceeded, engage the controlled clutch arrangement; determine from theturn sensor when the vehicle travels a substantially straight path, andapply the first level of the threshold value; determine from the turnsensor when the vehicle enters a driven turn, and apply the second levelof the threshold value, which second level is higher than the first; anddetermine at least partly from the turn sensor when the vehicle enters abraking turn, apply the service brakes of the front and the rear wheelsadjacent the inside of the turn and apply the third level of thethreshold value, which third level is higher than the second.
 10. Thedriveline as claimed in claim 9, further comprisingindependently-operable parking brakes on the rear wheels, wherein thecontrol system is configured to detect that the vehicle is entering thebraking turn, and additionally apply a predetermined level of brakingforce, to the parking brake on the rear wheels adjacent to the inside ofthe turn.
 11. The driveline as claimed in claim 10, wherein the controlsystem is configured to determine and apply a level of braking force tothe parking brakes of the rear wheels of the vehicle entering thebraking turn wherein, the applied braking force is substantiallyproportional to the level of braking force applied to the service brakesof the same rear wheels.
 12. The driveline of claim 9, comprising auser-operable brake control and sensor for determining applied pressure,wherein the control system is configured to set the level of brakingforce applied to the service brakes during a turn based on the level ofpressure exerted on the user-operable brake control.
 13. The drivelineof claim 9, wherein the control system includes an input for data and isconfigured to vary at least one of the first, second and third levels ofthe threshold value based on the input value for one or more factorscomprising the vehicle wheelbase, the vehicle track width, and a maximumsteering angle of the vehicle.
 14. A utility vehicle comprising thedriveline of claim
 9. 15. The utility vehicle of claim 14, furthercomprising a geographical positioning system coupled with the controlsystem, wherein the control system is configured to not implement brakesteering on determination that the vehicle is outside of a predeterminedgeographical area.